The present invention pertains low pressure airflow sensors. More particularly, it pertains to a sensor which can determine low pressure differentials.
Pressure sensors are utilized most commonly as pressure switches, turning on , as an example, electronic air cleaners when a pressure differential is determined. The switch compares the pressure downstream of the air cleaner to the pressure of the air surrounding the air cleaner. Usually, there is a return duct between the air cleaner and a grill to ambient air. When a return duct is present the pressure differential the airflow swtich senses is the pressure differential between the pressure following the air filter and the ambient air pressure.
An example of a prior art pressure swtich is U.S. Pat. No. 4,122,337, issued to Okuda et al. The pressure switch illustrated in Okuda et al. is a pressure electrical signal conversion means comprising a diaphragm assembly which is moved in accordance with the difference of pressure between the two compartment therein. Illumination means directs light in a band onto a photo-sensitive unit which includes two elements so disposed that when any particular band of the photo-sensitive area is illuminated, the illuminated area of one element is greater than that of the other. The photo-sensitive element produces an output proportional to the relative size of the illuminated areas of the elements. The photo-sensitive unit of an illumination means being moveable relative to one another in proportion to the movement of the diaphragm. Examples illustrate mounting a plate onto a diaphragm and preventing light from reaching a sensor through a shield until a specified pressure is reached. A second method shown illustrates attaching the sensor itself to the diaphragm, and determining the position of the diaphragm relative to where the light is reflected on the sensor. This method is useful, however, it is only useful for determining whether or not a set pressure has been achieved and cannot be utilized for determining what the pressure is.
A second application of a pressure sensor is in commercial building environmental control. This application differs from a pressure switch, as it can provide pressure indications over a continuous range. Most heating and cooling units divide the building into zones for heating and cooling. The zones will have pressure differentials between them which the heating and cooling unit must be able to sense. Pressure sensors can be utilized for this purpose.
SUMMARY OF THE INVENTION
In accordance with the principles of the invention an air pressure sensor is achieved which measures air pressure based upon the intensity of reflected light. The air pressure sensor is capable of sensing pressure differential at least as low as 0.020 inch WC (Water Column). The pressure sensor comprises a diaphragm, a housing with input ports, and an opto-reflective sensor. The diaphragm is mounted in the housing such that the first port and the second port provide differential pressures to either surface. For example one port may provide an ambient pressure and the second port will be utilized to provide the pressure to be measured. In this manner the diaphragm is forced toward the lesser of the two pressures. An optical reflector is mounted onto the diaphragm. The opto-reflective sensor is mounted opposite the optical reflector such that the opto-reflective sensor transmits an optical signal to the optical reflector, the optical reflector reflects the signal back to the opto-reflective sensor. The intensity of the reflected signal is depended upon the distance the optical reflector is from the opto-reflective sensor. Based on the intensity of the reflected signal it is then possible to calculate the differential pressure.
A compensation means is further disclosed in copending and commonly owned application entitled, "Compensation Mechanism for a Pressure Sensor", Ser. No. 816,026, filed on the same date and is hereby incorporated by reference.